T1 weighted brain images at 7 Tesla unbiased for Proton Density, T2* contrast and RF coil receive B1 sensitivity with simultaneous vessel visualization.
T1 weighted brain images at 7 Tesla unbiased for Proton Density, T2* contrast and RF coil receive B1 sensitivity with simultaneous vessel visualization.
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DOI：
10.1016/j.neuroimage.2009.02.009
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发表时间：
2009-06
期刊：
影响因子：
5.7
通讯作者：
Moeller S
中科院分区：
文献类型：
--
作者：
Van de Moortele PF;Auerbach EJ;Olman C;Yacoub E;Uğurbil K;Moeller S
在高磁场下，MR图像表现出大的，不希望的信号强度变化，通常称为“强度场偏置”。这种不均匀性主要源于不均匀的射频线圈B1剖面，并且在没有适当校正的情况下，当使用有根的平方和重构与接收线圈阵列时，这种不均匀性进一步明显。这些伪影可以显著改变全脑高分辨率t1加权（T1w）图像，这些图像广泛用于临床诊断、灰质/白质分割以及与功能时间序列的共配准。在T1加权的3D-MPRAGE序列中，可以通过使用绝热反转RF脉冲来保留大量的T1对比度，该脉冲对传输B1变化不敏感，高于最小阈值。然而，图像中仍然存在较大的强度变化，这在非常高的场中更加难以解决，因为RF线圈B1剖面变得更加不均匀。T1w MPRAGE序列的另一个特点是它们对质子密度和T2*对比度的固有灵敏度，这不能通过用于校正接收线圈灵敏度的后处理算法来消除。在本文中，我们展示了一种简单的技术，能够产生标准化，高分辨率的T1w 3D-MPRAGE图像，该图像没有接收线圈灵敏度，质子密度和T2*对比度。这些图像适合在7特斯拉下常规获得全脑组织分割，比标准MPRAGE获取具有更高的T1对比度特异性。我们的研究结果表明，去除质子密度成分可以帮助识别小的大脑结构，并且可以从图像中去除T2*诱导的伪影。由此产生的无偏T1w图像也可用于生成最大强度投影血管图，而无需额外的数据采集，这些血管图固有地与T1w结构图像注册。此外，我们还介绍了一种简单的技术来减少由传输B1异质性引起的剩余信号强度变化。因为这种方法需要两个3D图像，一个与另一个分割，头部运动可能会产生严重的问题，特别是在高空间分辨率下。为了减轻这种扫描间运动问题，我们开发了一种新的序列，其中两个对比采集在单个扫描中交错。然而，由于单次扫描时间较长，这种交错方法存在更大的扫描内运动问题风险。用户可以根据具体的方案和患者群体在这两种权衡之间进行选择。我们认为，这种基于双重对比度的方法的简单性和鲁棒性，可以解决高场强度场偏差和提高T1对比度特异性，以及同时获得血管造影图的能力，有利地抵消了该技术的潜在缺点，主要是较长的采集时间和适度降低信噪比。
At high magnetic field, MR images exhibit large, undesirable signal intensity variations commonly referred to as “intensity field bias”. Such inhomogeneities mostly originate from heterogeneous RF coil B1 profiles and, with no appropriate correction, are further pronounced when utilizing rooted sum of square reconstruction with receive coil arrays. These artifacts can significantly alter whole brain high resolution T1-weighted (T1w) images that are extensively utilized for clinical diagnosis, for gray/white matter segmentation as well as for coregistration with functional time series. In T1 weighted 3D-MPRAGE sequences, it is possible to preserve a bulk amount of T1 contrast through space by using adiabatic inversion RF pulses that are insensitive to transmit B1 variations above a minimum threshold. However, large intensity variations persist in the images, which are significantly more difficult to address at very high field where RF coil B1 profiles become more heterogeneous. Another characteristic of T1w MPRAGE sequences is their intrinsic sensitivity to Proton Density and T2* contrast, which cannot be removed with post-processing algorithms utilized to correct for receive coil sensitivity. In this paper, we demonstrate a simple technique capable of producing normalized, high resolution T1w 3D-MPRAGE images that are devoid of receive coil sensitivity, Proton Density and T2* contrast. These images, which are suitable for routinely obtaining whole brain tissue segmentation at 7 Tesla, provide higher T1 contrast specificity than standard MPRAGE acquisitions. Our results show that removing the Proton Density component can help identifying small brain structures and that T2* induced artifacts can be removed from the images. The resulting unbiased T1w images can also be used to generate Maximum Intensity Projection angiograms, without additional data acquisition, that are inherently registered with T1w structural images. In addition, we introduce a simple technique to reduce residual signal intensity variations induced by Transmit B1 heterogeneity. Because this approach requires two 3D images, one divided with the other, head motion could create serious problems, especially at high spatial resolution. To alleviate such inter-scan motion problems, we developed a new sequence where the two contrast acquisitions are interleaved within a single scan. This interleaved approach however comes with greater risk of intra-scan motion issues because of a longer single scan time. Users can choose between these two trade offs depending on specific protocols and patient populations. We believe that the simplicity and the robustness of this double contrast based approach to address intensity field bias at high field and improve T1 contrast specificity, together with the capability of simultaneously obtaining angiography maps, advantageously counter balance the potential drawbacks of the technique, mainly a longer acquisition time and a moderate reduction in signal to noise ratio.
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